Is it's cum and is this puberty
Yes it is cum.
It's not unusual for men to have thicker, more clumpy semen if it has been a while since last they ejaculated.
That is, the longer the build-up between climaxes...(days to weeks) the thicker the semen. It's therefore NOT unusual for a boy's first ejaculation to be extremely thick.
Relax. Nothing's broken. You're normal.
normal cum, youre 'of age'...congrats